The AF indicator lighting up probably means that your mum's heart rate was irregular at that point in time. Some patients have paroxysmal atrial fibrillation, which means that the atrial fibrillation comes on spontaneously from time to time. Apixaban is an anticoagulant (a medication that prevents blood clots) prescribed to patients with atrial fibrillation to prevent blood clots and strokes. Apixaban does not restore the heart rhythm to normal.
If your mum is feeling normal and has no chest pain, breathlessness or feeling giddy, there is probably no cause for concern. You can highlight this to her regular doctor at the next scheduled check-up.
If she has any of the mentioned symptoms, then she should see a doctor.